We have studied some of the initial interactions between the predacious bacterium Bdellovibrio bacteriovorus and its bacterial prey. Our primary interest has been how the predators recognize susceptible prey and what determines when a bdellovibro is committed to the predation. We have focussed during the past year on a characterization of the outer-membrane proteins of saprophytically grown bdellovibro (in the absence of host cells) and parasitically grown cells. We noted significant differences in the composition of the outer-membrane proteins of the cells grown in these different modes. We have grown the cells on different hosts (E. coli K-12 and some cell wall mutants and Salmonella typhimurium wild type and cell wall mutants) and have discovered that outer-membrane proteins of the host cells are incorporated into the bdellovibrio wall. We have also shown that these proteins are functional in terms of receptor activity. These studies should help us to explain the specificities of bdellovibrios, provide information of general interest concerning predator-prey and cell-cell interaction and developmental processes involving programmed sequences of events.